Innovative contaminant mass flux monitoring in an aquifer subject to tidal effects

Exposure from groundwater contamination to aquatic receptors residing in receiving surface water is dependent upon the rate of contaminated groundwater discharge. Characterization of groundwater fluxes is challenging, especially in coastal environment where tidal fluctuations result in transient groundwater flows towards these receptors. This can be further complicated by the high spatial heterogeneity of subsurface deposits enhanced by anthropogenic influence such as the mixing of natural sediments and backfill materials, presence of subsurface built structures or even occurrence of other sources of contaminant discharge. There is thus a need to address the risk posed by groundwater contaminations in coastal ecosystems using contaminant mass flux calculations based on direct continuous groundwater flux measurements able to capture the time variability of transient flows.

In this study, the Finite Volume Point Dilution Method (FVPDM) was successfully used to characterize highly transient groundwater flows and contaminant mass fluxes within a coastal groundwater flow system influenced by marked tides. The investigated aquifer is separated from a river by a sheet pile wall whose hydraulic effect was not fully characterized. The aquifer is constituted by materials of both anthropogenic and natural origins used as backfills. FVPDM tests were undertaken continuously for more than 48 hours at 6 groundwater monitoring wells, in order to evaluate groundwater flow dynamics during several tide cycles. Contaminant concentrations were measured simultaneously and allowed to calculate contaminant mass fluxes.

Groundwater and contaminant mass flux monitoring enabled a significant refinement of the conceptual site model. The study emphasized the high heterogeneity of the aquifer with groundwater fluxes ranging from 10-7 to 10-3 m/s. Groundwater fluxes monitoring showed no clear evidence of inversion of groundwater flow direction that would have resulted from river water infiltration in the aquifer. This is certainly due to combined effect of strong groundwater discharge from inland and of flaps present in the sheet pile wall preventing sea water intrusion in the aquifer when closed at high tide.

Although contaminant concentrations varies by one order of magnitude between different monitoring wells, contaminant mass fluxes are 7 to 3500 times higher in one particular monitoring well compared to other wells due to a combination of significant contaminant concentrations and high groundwater fluxes. Spatial variability of calculated solute mass fluxes allows to suggest that contaminant transfer within the aquifer should be considered as localized channel flows.

These results constitutes highly valuable information for optimization of further investigations and risk mitigation measures.